Beams of laser-radiation are increasingly used for cutting, drilling, marking, and scribing workpieces made of a wide range of materials; including metals, glasses, and polymers. Traditional mechanical processing produces unwanted defects, such as micro-cracks that may propagate when a processed workpiece is stressed, thereby degrading and weakening the processed workpiece. Laser-processing minimizes such unwanted defects, is generally cleaner, and causes a smaller heat-affected zone. Laser-machining uses a focused laser-beam to produce precise cuts and holes, having high-quality edges and walls, while minimizing the formation of unwanted defects. In laser welding, a focused laser-beam locates each weld spot or seam precisely and produces minimal collateral heating.
Fiber lasers are used extensively in industrial laser-processing applications that require both high power and high beam quality. For example, laser cutting and laser welding of metals and metal alloys. In a fiber laser, the gain medium is an optical fiber having a core that is doped with an optically-active ion, such as neodymium (Nd3+), ytterbium (Yb3+), thulium (Tm3+), or erbium (Er3+). A laser beam at an emission wavelength of the optically active ion is both amplified and guided within the core. The core is typically energized with pump radiation provided by a plurality of diode lasers. Diode lasers efficiently convert electrical power into optical power that can be directed into a gain fiber. In a “cladding-pumped” arrangement, the pump radiation is guided along the gain fiber in a pump cladding that jackets the core. An outer cladding jackets the pump cladding.
Most of the pump radiation is absorbed by the core over a sufficient length of gain fiber, which is desirable for optimal overall efficiency. Too much unabsorbed pump radiation can cause undesirable heating and even damage optical components located in the path of an output laser beam. By way of example, for a cladding-pumped gain fiber that absorbs 1 decibel (dB) of pump radiation per meter (m) length of fiber, 20 m of gain fiber is required to absorb 20 dB of pump radiation.
For high-power operation, especially in gain fibers having a small diameter core for single-mode operation, undesirable non-linear processes become enhanced through confinement of the amplified laser beam within the core. For pulsed laser operation with high peak-powers, non-linear processes such as four-wave mixing and self-phase modulation spectrally broaden the beam and reduce overall efficiency. For laser beams having narrow spectral bandwidths and long temporal coherence, stimulated Brillouin scattering causes back reflections that reduce efficiency and ultimately limit the output power of the fiber laser. Such back reflections can permanently damage the fiber laser and render it inoperative. Non-linear processes can be mitigated by reducing the length of the gain fiber. However, the concentration of the optically-active ion in the core must be increased to compensate and maintain overall efficiency, which increases the heat load on the gain fiber. For high-power operation, high heat loads can cause transverse mode instabilities that substantially degrade beam quality and mode stability.
For many optically active ions, particularly thulium, self-absorption causes the wavelength of maximum net gain to shift to longer wavelengths as the length of a gain fiber is increased. In a fiber laser resonator, resonator mirrors having relatively narrow spectral bandwidths are located at each end of the fiber laser resonator and are selected to force operation at a desired wavelength within the gain spectrum of the gain fiber. However, self-absorption causes a mismatch between the desired wavelength and the wavelength of maximum net gain in the gain fiber. This mismatch becomes a loss that ultimately clamps the power a laser beam can extract from the energized gain fiber. It also makes the operating wavelength of a fiber laser resonator sensitive to external back-reflections. Again, the wavelength mismatch and sensitivity to back-reflections could be mitigated by reducing the length of the gain fiber.
One way to maintain efficiency in a shorter gain fiber, without increasing the concentration of optically active ions in the core, is to recycle any unabsorbed pump radiation back into the pump cladding. This pump recycling works best in gain fibers that are pumped from just one end because the highest thermal loading is at the pumped end of the gain fiber. Unabsorbed pump radiation is recycled at the unpumped end of the gain fiber. Various recycling schemes have been proposed and tried. For example, Lang U.S. Pat. No. 6,477,295 has a groove around the circumference of the gain fiber. The groove has a surface inclined at 45° and must extend through most of the thickness of the pump cladding to efficiently recycle pump radiation, which mechanically weakens the gain fiber. Lang also proposes a high-reflection coating applied to most of an end facet of the fiber, with another etched fiber fusion spliced to the center of the end facet to guide the laser beam through the end facet. Again, this structure is mechanically weak. Depositing a uniform coating on such an assembly is difficult due to shadowing. Further, the coating having a relatively small area is vulnerable to optical damage when exposed to the unabsorbed pump radiation. Nikolajsen U.S. Pat. No. 6,700,697 proposes a high-reflection coating for both the pump beam and laser beam that completely coats a flat or hemispherical end facet. This coated end facet is similarly vulnerable to optical damage and is only useful in more complex multi-pass amplifier arrangements.
Starodoumov U.S. Pat. No. 7,286,283 describes a fiber amplifier having pump couplers at each end of the gain fiber. Each pump coupler includes multiple pump fibers that are bundled together and taper towards the gain fiber. Pump fibers that are not otherwise used may include mirrors to reflect unabsorbed pump radiation back to the gain fiber. Alternatively, unabsorbed pump radiation may be routed from an unused pump fiber in one coupler to an unused pump fiber in the other coupler. It should be noted that such rerouting would increase the thermal load at the hotter pumped end. Although these arrangements improve efficiency, only a fraction of any unabsorbed pump radiation is recycled, which limits the improvement in efficiency. Further, pump couplers having multiple bundled pump fibers are complex and difficult to fabricate.
There is need for a fiber laser architecture with a short gain fiber that efficiently recycles unabsorbed pump radiation and can be scaled to high powers. The architecture should be mechanically robust and resistant to optical damage. Preferably, the pump recycling would use relatively simple laser components, which are inexpensive and easy to fabricate.